In electronic apparatus having amplifier stages with relatively large power output capabilities, such as audio amplifier for example, a commonly encountered problem is a thermal run away condition of the amplifier device. Frequently, the idle current of the device is sufficient to cause this thermal run away condition which destroys the device.
One approach to solving the thermal run away problem is the use of a semi-conductor to sense a temperature change in the output device. Upon sensing the temperature change, the temperature sensor semi-conductor feeds back a signal which automatically controls the idle current of the output device and inhibits any tendency toward the run away condition.
In the prior art, a well-known technique for inhibiting thermal run away in an assembled power amplifier and temperature sensitive semi-conductor device, was to affix the power amplifier to the heat sink with a mica insulator between the heat sink and the amplifier. Also, the temperature sensitive device was affixed to the heat sink by a mounting bracket. Thus, the thermal path from the output device to the temperature sensitive device included (a) the case of the output device, (b) a mica insulator, (c) the heat sink, (d) through a portion of the heat sink, (e) a sensor device mounting bracket, and (f) the temperature sensitive device.
Obviously, such an extended thermal path includes numerous variables and the variables are different for each configuration. As a result, the response of the temperature sensitive device is different for each configuration and the time lag of the correction circuit may be of a length sufficient to cause failure of the power amplifier before the temperature sensitive device has had time to effect the correction.